The present invention relates to a new and improved method of concentrating and drying aqueous fluent media.
Oftentimes there exists the desire to evaporate to dryness suspensions, solutions, emulsions and the like. This operation is then particularly of interest when the media to be treated contains toxic agents or radioactive wastes. Nuclear power plants produce waste water, which, for instance, contains borates, alkali sulfates, for instance sodium sulfate, alkali silicates, tensides and the like. If such solutions or suspensions are evaporated to dryness, then generally they form scale. Such scale, particularly during continuous concentration and drying, for instance in a thin film rotational evaporator, leads to operating disturbances.
In Austrian Pat. No. 192,382 there is disclosed to the art a method of concentrating sulfite waste liquor or similar calcium sulfate-containing solutions, where there does not occur any scale formation. The formation of scale or incrustations is prevented by the addition of calcium sulfate slurry to the solutions. The calcium sulfate crystals serve as crystallization nuclei, so that there are formed a multiplicity of crystals and no scale is deposited upon the walls. A further refinement of this method has been described in Austrian Pat. No. 206,412, wherein with this disclosed method particular importance is place upon the good formation of the calcium sulfate crystals. Thus, the calcium sulfate crystals, prior to their addition to the solution containing calcium sulfate, are stored in water in order to obtain an orderly crystal formation.
With the above-discussed methods the crystallization nuclei are increased. As the crystallization nuclei there can be generally employed simply crystals which possess the same crystallization form as the substance to be separated out. To practice this method it is thus necessary to know what substance causes the scale formation, and then such substance is introduced in a crystalline state. However, if another substance also causes the formation of scale, then it is necessary to also add this substance in a crystalline state. The procedure needed to practice such prior art method are complicated and only accomplish the intended purpose if there is known the exact composition of the solution which is to be evaporated to dryness. In case of borates these procedures are not however effective, since borates, when they are evaporated to dryness, vitrify and thus form an extreme amount of scale.